Building each individual prosthetic remains an art, with amputees dependent on the craftsmanship and experience of individual designers, said David Moinina Sengeh, a PhD student at the Massachusetts Institute of Technology. Sengeh grew up among civil war victims in Sierra Leone, some of whom lost limbs; today, he is working to create a systematic process to produce comfortable, custom-fit prostheses every time.

His tools: magnetic resonance imaging, or MRI technology, used to map the limb to which a prosthetic device must fit, and a latest-generation 3-D printer that produces exact prototypes — solid objects from software designs based on precise MRI measurements.

The goal is to design a prosthetic socket that is flexible where the body is stiff to reduce pressure, and stiff where the body is flexible to provide support.

Sengeh has spent the past eight months printing out various versions of a prosthetic socket – the device that links a human body with an artificial limb – and testing them on his thesis advisor, professor Hugh Herr, a double amputee and the developer of a bionic ankle system.

The printer he uses is manufactured by Objet Ltd., which is based in Israel but has its North American headquarters in Billerica. The Objet machine is the only 3-D printer that can produce a single object containing both hard and soft materials.

It works much like a common inkjet, printing out one paper-thin layer at a time, each layer being partly cured by ultraviolet light before the next is sprayed down so the layers will fuse as they harden.

In April, Objet dropped a planned initial public offering of stock and said it would merge with Stratasys Inc., a publicly held manufacturer in Minnesota, to form a printer company worth $1.4 billion.

Its printers are suited for producing precise prototypes, but not the actual prosthetics, said Terry Wohlers, president of the industry consulting firm Wohlers Associates Inc., of Fort Collins, Colo., because the company’s photopolymer material is not yet strong enough to hold up to the daily pressures of the human body, or other taxing applications.

But fast, accurate Objet-formed prototypes already help a wide range of users, from shoe companies designing footwear to architects modeling structures in three dimensions.

The technology of 3-D printing has been available for more than two decades but is becoming cheaper, more versatile, and more precise.

Some printers manufacture products from a single material. More than 15,000 Americans are walking around with hip implants designed in software form, then printed on a 3-D printer, Wohlers said. Other applications include such demanding products as parts for airplanes.

“That’s where this product is best used, where you have complex, high-value parts,” Wohlers said. “Companies are beginning to move from a tradition of having physical inventories, shelves of parts, to having digital inventories, where parts are stored digitally and printed on demand.”

In the past, “you’d be waiting for machine shop, or waiting for parts to come back from China before you could evaluate them,” said John Cogswell, Objet team lead applications engineer. “Now, you’re able to get that whole design [into your hands] in a matter of hours.”

He uses an Objet printer for jobs that require finer details and mixed materials. “You don’t have to worry about assemblies of parts, because you can build everything as one part,” he said of the Objet printer.

At MIT, Sengeh is working to produce high-quality sockets that will be inexpensive enough to outfit amputees he knows in Sierra Leone. Their futures are extremely limited, he said, because they don’t have prostheses that are comfortable enough to wear daily.

“Imagine if they gave you shoes that were two sizes too small,” he said. “If it’s uncomfortable, it doesn’t matter if you have the most advanced prosthetic in the world.”

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